H. H. Klauß

Magnetic order in NpO2 and UO2 studied by muon spin rotation

Abstract Muon spin rotation/relaxation (μSR) measurements were carried out in zero applied field on NpO 2 and UO 2 above and below their transition temperatures of 25 and 30.8 K, respectively. In NpO 2 , a spontaneous spin precession pattern is observed for T 2 where the 30.8 K transition is well established as the Neel temperature for antiferromagnetic order. The μSR patterns are, in their basic features, quite alike for the two compounds. This establishes unambiguously that the transition at 25 K in NpO 2 is basically of magnetic origin. The ordered moment on Np is estimated to be smaller than about 0.15 μ B , i.e. less than 10% of the moment on U in antiferromagnetic UO 2 .

High pressure μSR studies

The high pressureμSR spectrometer [1] formerly located at CERN has been transferred to theμE1-beamline at PSI and put back into operation with only minor modifications. The essential features of the high pressure apparatus are described below. The instrument covers a pressure range up to 0.7 GPa which can be extended to 1.4 GPa depending on the design of the high pressure cell. First measurements at PSI were successfully carried out with a single crystalline sample of Gd metal. New developments in high pressure cell design are presented. They are expected to further improve the signal/background ratio and to extend the pressure range to 1 GPa. One type of cell will allow temperatures above 380 K.

Fast paramagnetic relaxation in Mössbauer spectra of ferrocenium salts

We performed 57Fe Mössbauer spectroscopy on differently substituted ferrocenium salts. The spectra exhibit a temperature dependent non-Lorentzian absorption line due to fast paramagnetic relaxation processes. χ2 fits using a two-level stochastic fluctuation theory revealed an activated temperature behaviour of the relaxation rate interpreted in terms of an Orbach process.

μSR on the novel heavy‐fermion system Nd2-yCeyCuO4

The magnetic correlations in Nd2-yCeyCuO4 have been studied by zero and longitudinal field μSR. Nd1.8Ce0.2CuO4 reveals a saturation of the muon relaxation rate below 0.5 K. Even down to 70 mK LF spectra evidence spin fluctuations in the dynamic regime with a rate of \sim 109 s-1 excluding long range magnetic order or spin glass freezing. The average Nd moment is estimated to be \approx 0.2\muB, i.e., strongly reduced from the value determined for the ground state Kramers doublet of Nd2CuO4. Extending former μSR measurements on Nd2CuO4, a gradual enhancement of the internal field has been detected below 10 K which is accompanied by an increase of the relaxation rate. These results are attributed to the development of ordered Nd moments.

Valence fluctuations in ternary europium compounds

We outline the possibility to study europium valence fluctuations with the μSR method and report on μSR experiments on the intermetallic compounds EuPdAs and NdPdAs. Above a magnetic transition at 15 K the temperature dependence of the relaxation rate in the trivalent neodymium system behaves like a typical localized moment system. In the valence fluctuating europium compound the zero field relaxation rate levels off at 1.0\ μs-1 above 40 K. Furthermore, the relaxation enhancement in transverse field experiments is much smaller than expected for a pure dipolar coupling. Therefore an isotropic hyperfine coupling of typical strength is assumed and a valence fluctuation rate of 0.8 μs-1 at 200 K is derived. Below the magnetic transition at 5 K a disordered spin freezing is concluded in EuPdAs.

Magnetic features of the Kondo system CeTSn (T=Ni, Pd, Pt) probed by positive muons

Zero-, longitudinal and transverseμSR measurements are reported on CeNiSn, CePdSn and CePtSn. Below 1 K CeNiSn exhibits strong magnetic correlations, up to possible short range order, but a magnetic phase transition did not occur down to 0.033 K. CePdSn and CePtSn signal their transition into the ordered antiferromagnetic phase by a spontaneous spin precession signal. The second magnetic phase transition in CePtSn reveals itself by the onset of a more complex spin precession pattern. Most unusual is the formation of a paramagnetic state with frozen random spin structure in CePtSn about one Kelvin aboveTN.

μSR magnetic response in frustrated antiferromagnets of type RMn2 (R = rare earth)

Zero longitudinal and transverse fieldμSR was carried out in the antiferromagnets YMn2, Y0.95 Tb0.15 Mn2, Y0.9Tb0.1Mn2, Y0.99 Sc0.01 Mn2, Y0.98Sc0.02Mn2 and TbMn2. The dynamics of Mn magnetic moments above TN is typical for an itinerant antiferromagnet. Within a certain temperature range above TN part of the material enters a randomly ordered (spin glass like) magnetic state as an outcome of frustration. At temperatures above ∼ 150 K the muon spin relaxation rate indicates that the muon has become mobile.

Magnetic properties of CeSb and DySb from μSR

ZFμSR experiments on CeSb and DySb reveal fast spin dynamics even in their magnetically ordered states below 16.2 and 9.5 K, respectively. Above these first order transitions both pnictides exhibit strong frequency shifts in TF pointing to some magnetic precursors. The signals below TN show spontaneous rotating contributions. For CeSb several commensurately modulated antiferromagnetic phases containing paramagnetic sheets are confirmed. For DySb our data favor a CoO type structure in contrast to the MnO structure proposed from neutron diffraction.

The spin turning in ferromagnetic Gd studied by positive muons

We have studied the muon precession frequency in a ferromagnetic single crystal of Gd metal. The overall features of our findings are compatible with earlier results on polycrystalline material. In the temperature region between 245 and 220 K where the Gd magnetization starts to turn away from the c-axis, we observe an increase in the muon depolarization rate, and a complex precession signal which can be separated into two frequency components meaning that spin turning does not occur simultaneously in different parts of the sample (domains). From these more detailed data follows that previously obtained values forBfc andBdip can not both be correct. Two explanations for our new result are possible: EitherBfc undergoes a change around 230 K which is directly coupled to the spin turning angle, or the value of the dipolar field contribution used in the earlier evaluation is too low. This imposes some uncertainty as to the value of the angle at the onset of spin turning derived fromμSR frequencies.

μSR studies of uranium compounds with AuCu3 structure

We carried out 220 G transversal field, zero field and 2, 5 and 10 G longitudinal fieldμSR experiments on USn3, UIn3 and U(Sn0.5In0.5)3 between 300 K and 13 K. USn3 behaves analogous to the well known spin-fluctuator UAl2. UIn3 exhibits in its antiferromagnetic state (TN=88 K) a Lorentzian Kubo-Toyabe pattern of the type seen in other simple cubic uranium compounds like UAs. U(Sn0.5In0.5)3 does not order magnetically. Its μSR spectra at low temperatures suggest that this material should be considered a spin glass like system with disordered spin freezing setting in around 30 K.